The present invention relates to a process of removing hydrogen sulfide from natural gas or an industrial gas, in an integrated system wherein sulfur is produced. More preferably, the present invention relates to such processes wherein the gas being treated is under a relatively high pressure.
One of the most common systems for processing natural gas containing hydrogen sulfide and producing sulfur involves the use of well-known absorber-stripper steps to separate H2S and the well-known Claus process to produce sulfur. In such system, in simplified form, the basic steps are usually:    (a) H2S removal from sour gas, using an H2S absorbent, to obtain sweetened product natural gas;    (b) stripping H2S out of the H2S-rich absorbent to obtain H2S;    (c) H2S combustion to obtain a mixture of SO2 and H2S;    (d) solid-catalyzed H2S reaction with SO2 at high temperature to form and recover S and to make an off-gas containing reduced amounts of H2S and SO2; and    (e) treating the off-gas from step (d) to recover as S a major fraction of the remaining amounts of H2S and SO2 and to form a stack gas that is released to the atmosphere;Steps (c) and (d) in combination are often regarded as the Claus process.
My U.S. Pat. No. 6,495,117, which is hereby incorporated herein by reference, describes a process in which gaseous hydrogen sulfide (H2S) reacts with gaseous sulfur dioxide (SO2) in the presence of an organic liquid or solvent, preferably above the melting point of sulfur, wherein the following reaction occurs:2H2S(g)+SO2(g)→3S(l)+2H2O(g)  (1)
In that reaction, H2S is present in excess, so that an H2S-containing off-gas is produced. The reacting gases may flow co-currently or counter-currently to a stream of the organic liquid. A preferred example of such a reactor is a tray-type column in which the reacting gases flow counter-currently to a stream of the organic liquid. The sulfur produced by Reaction (1) in either type of reactor forms a separate liquid phase that flows co-currently with the organic liquid.
The gaseous sulfur dioxide is produced by combustion of hydrogen sulfide contained in the reactor off-gas. Preferably this combustion is conducted fuel-rich to avoid the risk of forming SO3 and NOx, both of which are undesirable.
My U.S. Pat. No. 6,645,459 discloses a generally similar process in which SO2 gas, en route from the combustion furnace to the reactor, is cooled using a direct or indirect quench to produce an aqueous stream that is introduced into the reactor at one or more points to provide cooling.
In other known processes, H2S and SO2 are present in the reactor in stoichiometric or near-stoichiometric quantities. For example, a process piloted by the Jefferson Lake Sulfur Company but never put into commercial operation [Oil and Gas Journal, 50(4), pg. 59, 1951] burns sulfur to obtain SO2 that is then mixed with an H2S-containing gas. The H2S and SO2 are in stoichiometric ratio. The mixture is heated and passed over a series of catalytic beds similar to those used in the conventional Claus process.
A process described in U.S. Pat. No. 3,170,766, (Townsend) generates SO2 by burning sulfur with air and absorbing the SO2 in di- or triethylene glycol and contacting an H2S-containing gas with the solution at near-ambient temperature to generate a slurry of solid sulfur in the glycol. The slurry is then heated above the melting point of sulfur, settled, and the two liquid phases are separated by decanting. Water and unreacted SO2 are separated from the glycol phase by distillation. A major disadvantage of this process is the need to cool the sulfur-saturated, regenerated glycol below the sulfur-precipitation temperature. Solid sulfur will coat cooling surfaces under these conditions if a heat exchanger is employed.
In a process described in U.S. Pat. No. 3,441,379 (Renault, assigned to the Institut Francais du Petrole and commercialized as the “IFP process”) H2S and SO2 react in a column in the presence of a solvent consisting of ethylene glycol, water and a catalyst at a temperature above the melting point of sulfur. The IFP process is employed to treat the tail gas of a Claus plant and hence the H2S and SO2 are present, to the extent possible, in exact stoichiometric ratio—neither is in excess and hence neither can be substantially reacted away. As a result the gaseous effluent from the IFP reactor contains objectionable amounts of both H2S and SO2. The process still requires incineration of the tail gas to eliminate H2S, and the SO2 content of the stack gas cannot meet today's strict environmental standards.
Another process, the CrystaSulf™ process, is described in U.S. Pat. Nos. 5,733,516 and 5,738,834 (DeBerry) In it, H2S reacts in a column at high pressure in the presence of a non-aqueous, water-immiscible organic solvent consisting of one or more tertiary amines to provide basicity and one or more aromatic solvents to render the solvent immiscible in water. The reaction is carried out at a temperature high enough, typically 50° to 70° C., to keep the sulfur formed in solution. The sulfur-rich solvent is then cooled to cause a part of the sulfur to precipitate by crystallization. The oxidizing power of the solvent is restored, either with air or with SO2, before the solvent is recycled to the H2S-absorption step. Because of the basicity of the CrystaSulf solvent, 10% or more of the H2S absorbed is converted to sulfate or thiosulfate, which must be removed continuously from the solvent with a caustic wash step. Costs are incurred for the caustic and for disposal of the salts formed. Because the solvent is water-immiscible, the sulfur product must be washed with methanol or other volatile solvent to clean the sulfur and to recover Crystasulf solvent. Both of the latter must then be treated further to reclaim the volatile solvent mixed with them. Still another complication of the CrystaSulf process is the need to cool the sulfur-saturated solvent in a heat exchanger to cause sulfur to precipitate. Solid sulfur coats the cooling surfaces and must be removed on a regular basis.
Another process is disclosed in U.S. Pat. No. 4,124,685 (Tarhan). Here again, an excess of H2S with respect to SO2 is used in the reactor, to ensure that the reactor off-gases do not contain SO2. However, the H2S in the reactor off-gas is recycled rather than combusted. SO2 for the reaction is produced by combustion of sulfur, particularly some of the sulfur produced in the reactor.
Current processes for treating gases having low concentrations of H2S tend to involve sequestering the H2S in a chemical that is discarded, creating waste problems, or using an aqueous redox process to form a colloidal slurry of sulfur, that includes a complex regeneration system, or require an amine absorber/stripper system to concentrate the H2S before it is fed to the reactor.
Improvements to and variations in the Claus process are still being made. There is, however, still a need for a process for sulfur removal from gases, especially industrially generated gas streams, that meets high standards for emissions controls, is suitable for use with gas streams containing relatively small amounts of H2S, recovers sulfur values, is economic, and is versatile. In addition, there is a need for a process having the capability of treating gases at higher pressures, where reaction rates are increased, and where solvent flows can be reduced. This invention provides such a process.